Mobs are only loading in at very short range. The video is taken in 1.12.2, but I was playing on 1.13 before and it did the same thing. In 1.13 it was without any extra add-ons, and I'm pretty sure every video option was set to max. In the video, I was using optifine and with all settings maxed out. You can also see that the terrain in the background hasn't loaded in to proper range. I have it set to load 32 chunks
Mob Rendering Range 1.12.2(at least)+